In an image forming apparatus of an electrophotographic system (including an electrostatic recording system), such as a copying machine, laser beam printer or facsimile, a developer is used for making an electrostatic latent image formed on a photosensitive member visible. The developer comprises, as a main component, colored resin particles with a colorant, a charge control agent, a parting agent and the like dispersed in a binder resin. The colored resin particles are called a toner or toner particles. A one-component developer with an external additive such as fine silica powder attached to the surfaces of the colored resin particles for improving flowability; or a two-component developer composed of the colored resin particles and a carrier may also be called a toner simply. Thus, when these developers are called toners, that fact is clearly described in this description.
The colored resin particles (toner) are roughly divided into a pulverized toner obtained by a pulverization process and a toner obtained by a wet process. In the pulverization process, colored resin particles (pulverized toner) are obtained by a process, in which a thermoplastic resin is melted and kneaded together with additive components such as a colorant, a charge control agent and a parting agent, and the resultant kneaded product is pulverized and classified. The thermoplastic resin used in the pulverization process is synthesized by polymerizing a polymerizable monomer and is a resin component containing no additive component therein.
In the wet process, colored resin particles (colored polymer particles) are obtained by, for example, a process (hereinafter referred to as “a suspension polymerization process”), in which a polymerizable monomer composition containing a polymerizable monomer and additive components is suspension-polymerized in the presence of a polymerization initiator in an aqueous dispersion medium. The colored polymer particles are called a polymerized toner. In addition to the suspension polymerization process, an emulsion polymerization aggregation process, a dispersion polymerization process and a dissolution suspension process are known as the wet process. For example, in the emulsion polymerization aggregation process, colored resin particles are obtained by a process, in which emulsion particles obtained by emulsion polymerization of a polymerizable monomer and various additive components such as a colorant are aggregated and granulated.
In the production process of toner particles by such a wet process, a drying step is arranged because water is used as a dispersion medium. In the production process of a toner by the wet process, a drying method by a continuous system (continuous process) or batch system (batch process) has heretofore been generally used.
In the drying method by the continuous system, a method making use of a flash dryer or fluidized bed dryer has been proposed. For example, Japanese Patent Application Laid-Open No. 2004-258589 (Patent Literature 1) has proposed a method of continuously drying toner particles with an air current by a loop type dryer in a drying step in a production process of a toner, including a step of washing and dehydrating toner particles formed in an aqueous dispersion medium and then drying the resultant wet toner particles. The mere use of the air current is insufficient in dispersion of the toner particles, and so drying efficiency is low. Patent Literature 1 describes that the toner particles were dried with hot air controlled to 90° C. (see Example 1 and the like). When the drying step is performed by using the hot air controlled to 90° C., the drying efficiency can be improved, but fusion bonding of the toner particles to an inner wall of the drying device cannot be avoided, and moreover the toner properties of the resulting toner tend to be lowered due to thermal fusion bonding among the toner particles and deterioration of the toner particles at the high temperature. When the temperature of the hot air is lowered, toner particles that are not sufficiently dried come to pass through the drying device. In other words, undried toner particles short-pass through the drying device.
Japanese Patent Application Laid-Open No. 11-184153 (Patent Literature 2) discloses a method, in which colored polymer particles in a wetted state are continuously fed into a dryer with an agitating rotor and an inlet port of hot air arranged at lower portions thereof, and dried by forming a fluidized bed with the hot air while agitating the wet colored polymer particles by the agitating rotor. According to this method, the short-pass of the undried wet colored polymer particles can be prevented. According to the drying method disclosed in Patent Literature 2, however, the temperature of the hot air at the inlet port is controlled within a range from 60 to 150° C., preferably from 80 to 120° C. to conduct the drying treatment, so that fusion bonding of the colored polymer particles to an inner wall of the dryer is easy to occur when the dryer is operated for a long period of time, and a tendency to lower toner properties due to deterioration by heat is also shown.
On the other hand, as the drying method by the batch system (batch process), have been proposed a method making use of a conical type or Nautor type dryer and a vacuum-drying method. However, the conventional drying methods of the batch system takes a markedly long time to conduct drying compared with the drying methods of the continuous system and are thus extremely low in productive efficiency.
Therefore, there has been a demand for development of a drying method that prevents the deterioration by heat of colored resin particles (toner particles) and has high productive efficiency.    Patent Literature 1: Japanese Patent Application Laid-Open No. 2004-258589    Patent Literature 2: Japanese Patent Application Laid-Open No. 11-184153